I have taken a different approach, concentrating on the CHARGE on each
capacitor plate. For simplicity I assume ideal capacitors (no fringing), of
equal value. Once the EMF is disconnected, the charge +Q is trapped on the
upper plate of C1. For an ideal capacitor, a charge -Q will be attracted
onto the lower plate of C1 from wherever it is available. Grounding this
lower plate does not affect this charge value, and the potential across C1
is still 1/2 the original EMF, but it does lower the point V2 to ground
potential.